MARCO ISLAND, Fla.  Scientists are tracking a 400 square-mile bloom of red tide algae lurking off the Gulf Coast, pointing to it as the likely cause of a mass fish kill and several dolphin deaths at the tip of the Florida peninsula.

"It looks almost the size of a small state. It really is a large area of bloom," said Billy Causey, superintendent of the Florida Keys National Marine Sanctuary. "I'd have to say that this is the largest red tide bloom that I have ever seen."

A red tide is considered a higher-than-normal concentration of a plant-like microscopic algae. The species that usually contaminates the water during Florida Gulf Coast red tides is called Karenia brevis, which produces a toxin that can affect a fish's central nervous system.

It also produces a toxin in shellfish, which, if eaten, can cause a person to suffer tingling in the mouth and fingers, loss of coordination, hot and cold flashes and diarrhea.

Scientists first noticed the bloom in early November about 40 miles off the coast of Naples. Tests showed low to medium levels of red tide, said Merrie Beth Neely, marine research associate at Fish and Wildlife Research Institute, an arm of the Florida Fish and Wildlife Conservation Commission.

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Commercial fishermen reported fish kills, dead crabs and four dolphin carcasses this weekend. Some people have reported dead grouper as large as 26 inches.

Scientists with the National Oceanic and Atmospheric Administration and the Fish and Wildlife Research Institute plan to test water this weekend to track the bloom. They said it appears to be moving southward away from the Fort Myers-Naples coastline.